The present invention relates to a differential drive with an externally variable controllable locking coupling, especially for motor vehicles, having a drivable differential carrier rotatably supported in a housing, output gears arranged coaxial and retatable relative to the differential carrier and non-rotatable relative to their respective output shafts, further having differential gears simultaneously engaging the output gears and rotatably held in the differential carrier, as well as a friction assembly alternately comprising outer plates nonrotatingly connected to a first of the coaxially positioned parts (differential carrier and output gears) and inner plates nonrotatingly connected to another of the coaxially positioned parts (differential carrier and output gears), with these on the one hand being axially supported on a supporting face of the first of the parts (differential carrier or one of the output gears) and, on the other hand, being loadable via an actuating device and a pressure plate.
There are prior art externally controlled, lockable differential drive, in the case of which the friction assembly is loaded via hydraulic cylinders. These are integrated into the differential drive. Admittedly, this method of actuating lockable differential drives is relatively advantageous from the point of view of functioning and behavior, but the hydraulic components make it expensive. This applies even in those cases where a hydraulic system is already available for the power steering and suspension systems.
Furthermore, electromagnetic methods of actuating the friction assembly of a lockable differential drive have been proposed (DE-OS 37 07 115). However, the disadvantage of such an assembly is that a mechanical transmission between the coil and the friction plates achieves an acceptable locking effect within only a narrow wear range of the plates. This is due to the greatly decreasing actuating force of the coil if it does not remain in its optimum position relative to the armature.
There is a further prior art differential drive in the case of which a friction plate assembly is actuated electromagnetically (DE-OS 37 33 771). Its achievable locking effect is not high enough for it to be used as an axle differential and its design is too bulky for this purpose.
Finally, a design using an electric motor together with a reduction gearing system and/or expander plates working against a friction assembly comprising outer and inner plates is known (U.S. Pat. No. 4,805,486). This suffers from two technical disadvantages. Most importantly, the expander system is self-locking which prevents the friction assembly from unloading should there be an electrical failure when the system is locked up. This could give rise to a dangerous vehicle stability condition when the vehicle is moving. Secondly, simultaneous axial displacement and rotation of the gearing between expander plate and driving pinion can create additional friction and hysteresis in the modulation of the friction disc assembly by the actuating device.